Researchers at NASA’s Armstrong Flight Research Center have developed and tested a new strain gage that makes significant strides in the state of the art, particularly salient given the requirements of new structural components on aerospace vehicles. Conventional foil technology presents a significant shortcoming for these vehicles, since it is limited to less than 20 percent strains while newer vehicles include highly elastic, low-Young’s-modulus materials that require higher strain measurements. For example, fabric-reinforced rubbers and elastomers have a nonlinear stressstrain relationship with extreme rupture strains — some greater than 500 percent.

The sensor is primarily constructed of an indium-gallium liquid metal (LM)-filled, small-diameter silicone tube with electrical lead wires attached.
NASA Armstrong’s technology is based on a medical plethysmography sensor that circumferentially measures vascular flow in a limb or a finger. The sensor is primarily constructed of an indium-gallium liquid metal (LM)-filled, small-diameter silicone tube with electrical lead wires attached. When the LM is excited, length changes can be determined by typical strain resistance changes over the initial resistance.

The modified version of the original sensor lays flat and attaches to a test substrate in a single-looped strain gage configuration with minimal adhesive to minimize stiffing on low-modulus material. A simple tool reduces initial resistance scatter between gages and provides consistency in end-loop radii for conformity of transverse sensitivity. A new 24-bit circuit design incorporated the excitation current required for a range of 1.5 million microstrain (i.e., 150 percent strain), with a step resolution of less than 10 microstrain, and was designed to compensate for changing temperatures in varying thermal environments. Taking advantage of constant current makes it possible to derive strain using accurate initial resistance measurements (Kelvin) and plugging them into the strain equation. A data acquisition system processes strain equations and eliminates the need for two-point tensile calibrations.

The strain gage is highly beneficial for aircraft, aerospace materials, and other applications, including elastomer skins for highly flexed wing and control surfaces; high-cycle, high-strain fatigue testing; rubberized fabrics; cargo-carrying airships; inflatable wing-morphing aircraft; aeroservoelastic control; and flexible wind turbine blades.

NASA Armstrong’s sensors were laboratory tested under both bending and single-axial tensile modes against conventional foil strain gages as reference. Aluminum, Plexiglas®, and fiberglass materials were used for bending, and tensile testing was conducted on graphite-epoxy load bars to 10,000 microstrain. Follow-up sensor testing against photogrammetry technology to greater than 100 percent strains on elastomers demonstrated excellent repeatability and accuracy with negligible stiffening. These tests indicate that when used with specifically designed constant current signal conditioning, accurate static strain measurements are achievable for ground testing.

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This work was done by Anthony Piazza and Allen Parker of Armstrong Flight Research Center. For more information, contact the NASA Armstrong Technology Transfer Office at DFRC-TTO@ Refer to DRC-014-003.

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